In a typical blow molding machine, a screw extruder delivers molten plastic to an injection molding station to form the plastic preforms. These preforms may then be removed, transferred, and stored for subsequent reheating and blowing operations to form the final containers.
Alternatively, the preforms may, while still in the blow molding machine, be heated appropriately and blown to their final shape. This type of injection blow molding machine is a one step injection stretch blow molding machine. This machine produces containers in an uninterrupted fashion from raw stock to completely finished products. This is in direct contrast to the two step "reheat and blow" process first described. The one step process retains some of the latent heat in the injection preforms allowing the containers to be stretched and blown without reheating them from a cold condition. This saves energy. In addition, the preforms remain oriented in the machine throughout the entire machine cycle.
The preforms, once formed, must be transferred to a stretch and blow station for final processing. In a typical one step stretch blow molding machine, the preforms are delivered to a blow molding carousel or table which communicates with several other stations positioned around the carousel. At these stations, the preforms are first tempered to achieve optimum temperature conditions. The preforms are then delivered to a stretch and blow station at which blowing fluid is introduced to cause the preforms to expand to form containers of the desired final shape. The containers are then cooled and removed from the carousel.
While blow molding machines of this type are effective in forming blow molded containers, the capacity of such machines is limited by the through put of the injection molding operation. Because the duration of the injection molding operation is often a function of the cooling time of the plastic preform in the injection mold, and therefore is relatively constant, it is often not possible to increase the through put of the injection molding operation to match the through put capability associated with the other stations. Without changing the through put of the injection molding portion, any change in the speed of the other operations would not increase the capacity of the one step blow molding machine. In addition, because the screw extruder does not deliver the molten plastic to the injection molding machine while the preforms are being cooled, the molten plastic may heat excessively in the extruder. Thus the injection molding step of the machine cycle takes the longest time. It is the limiting factor in the production process.
Adding a second injection mold and alternately feeding preforms through the subsequent stations can effectively double the output of containers in a given time for the carousel type machine. In the present invention, the single injection mold machine has been redesigned to accept two injection mold stations, working alternately. The duel mold assembly machine utilizes a hydraulically actuated melt diverter valve to direct melt alternately to either the left hand or the right hand injection mold assemblies. Since there are now two injection mold assemblies, a unique preform transfer mechanism according to the present invention is utilized to receive molds or preforms from the injection mold assemblies and loads them onto chucks or holders carried on the carousel portion of the machine.
It is therefore the primary object of the present invention to increase the capacity of blow molding machines of the one step type.
It is a further object of the present invention to provide a transfer mechanism for transferring preforms between the mold cavities and appropriate stations on the carousel.
These and other advantages of the present invention will become apparent by a consideration of the following detailed description of the invention when taken in conjunction with the attached drawings and appended claims.